FIG. 4 shows a conventional starter which uses a motor for starting an engine. In the figure, a crankshaft 1 is provided with a fixed driven gear 3. The driven gear 3 is covered with a gear case 7 fixed to a crank case 5. A starter motor 9 is fitted to the gear case 7. The starter motor 9 has a rotary shaft 11 whose periphery is threaded to provide a screw portion 13 on which a drive gear 15 is movably fitted. The drive gear 15 removably engages with the driven gear 3.
When the starter motor 9 is actuated, the drive gear 15 tries to be stationary due to its inertia so that the drive gear 15 rotates relative to the rotary shaft 11. As a result, the drive gear 15 axially moves due to the screw portion 13 and engages with the driven gear 3 to rotate the crankshaft 1 to start the engine.
After the engine is started, a circumferential speed of the driven gear 3 exceeds that of the rotary shaft 11 so that the drive gear 15 rotates reversely relative to the rotary shaft 11. As a result, the drive gear 15 separates from the driven gear 3 and returns to its original position due to the screw portion 13.
According to this conventional arrangement, the mechanism for axially moving the drive gear 15 is complicated, and the gears may be injured due to shocks caused by the moving drive gear 15. Further, the driven gear 3 of large inertia fixed to the crankshaft 1 shall always be rotated together with the engine so that the crankshaft and bearings shall have sufficient strength. In addition, the rotary shaft of the starter motor and the gears tend to be injured because the driven gear 3 of large inertia is suddenly rotated by the engagement with the drive gear 15.